The overall goal of this study is to discover the representations of acoustic features by medial geniculate body (MGB) neurons in an awake primate. In doing so, we can ascertain how the known cortical representations of these features arise from the MGB representations, given the comparatively large body of knowledge regarding responses of auditory cortex neurons to auditory stimuli. Some deficits observed in humans with dyslexia and schizophrenia have been correlated with abnormalities in the MGB. An understanding of normal MGB responses to stimuli that produce abnormal responses in affected people, such as rapidly changing or multi-frequency stimuli will provide insight into the likely causes of neuronal dysfunction. We will study how sound characteristics are encoded by MGB neurons by recording the neural responses of single neurons in response to auditory stimuli. The recordings will be performed in awake marmosets, which are primates that possess a large repertoire of vocalizations that are used to communicate. Since auditory cortex responses have been studied extensively and are generated largely by integrating inputs from the thalamus, we will use auditory stimuli that have been used for auditory cortex studies, such as repetitive clicks, two-tone complexes, and amplitude-modulated tones. Knowing both thalamic and cortical responses, the ways that MGB inputs are combined to form new auditory cortex response properties are likely to reveal relevant acoustic features used for auditory perception. The specific aims of the study are as follows: 1) we will test the hypothesis that MGB neurons represent temporally modulated stimuli in a manner that is intermediate between that of the inferior colliculus and the auditory cortex, using separate populations of neurons. One population fires discharges that are locked to the repeated stimulus cycles in a synchronized manner and another population responds in a non-stimulus-synchronized manner through increases in discharge rate. 2) We will investigate the spectral context selectivity of MGB neurons. Using two-tone stimuli, we will test the hypothesis that frequencies that are harmonically related to a MGB neuron's best frequency will inhibit the neuron, whereas the two-tone facilitation observed in the auditory cortex will be weak in the MGB. 3) We will test the hypothesis that the contrasting synaptic and anatomic characteristics of two putative populations of IC excitatory inputs will lead to contrasting responses to tone, two-tone, click, filtered noise, and sinusoidally modulated stimuli in vivo. These two groups of contrasting responses will be differently localized within the MGB.